1. Field of the Invention
The present invention relates to an image signal processing apparatus for causing an image sensor such as a CCD to read an original image to obtain an image signal and processing the image signal.
2. Related Background Art
A conventional digital copying machine includes a known laser beam printer as an image recording output unit which utilizes an electrophotographic technique and a scanner as an image reading unit which causes a line sensor such as a CCD to photoelectrically read an image in a main scanning direction. Reading of the image along the sub-scanning direction in the scanner is performed by mechanically moving an original relative to the photoelectric transducer element in a direction perpendicular to the read direction of the element.
When an image output operation with a variable magnification is performed in the arrangement described, it is very difficult to stably change a scanning speed of main laser scanning in the axial direction of a photosensitive body in a laser beam printer and a scanning speed of rotation of a sub-scanning drum in a direction perpendicular to the axial direction of the photosensitive body. Therefore, variable magnification operations are performed on only the scanner side.
The original scanning speed of the scanner is increased relative to the rotational speed of the drum to reduce the image and is decreased relative thereto to enlarge the image. An image enlargement in the main scanning direction is performed by thinning a one-line image signal in the main scanning direction every predetermined number of pixels. An image reduction is performed by overlapping the one-line image signal every predetermined number of pixels.
In addition, edge emphasis is performed to emphasize an edge of a read image to obtain a sharp image. As a typical example of edge emphasis, a quadratic differential is performed by a Laplacian filter in both the sub-scanning and main scanning directions, and a pixel of interest is corrected on the basis of the quadratic differential.
FIG. 1 is an arrangement of a conventional edge emphasis circuit. A one-line digital image signal 801 is stored in each of line memories 820, 821, and 822 of a three-line delay buffer memory 802. A three-line image signal consisting of a current line image signal 803, an immediately preceding line image signal 804, and an image signal 805 of a line which is two lines ahead of the current line are output from the buffer memory 802. These image signals are delayed by a latch 806 in units of pixels.
The pixel of interest is a signal 807 obtained by delaying the immediately preceding image signal 804 by two pixels. An adder 811 adds a result obtained by doubling the signal 807 of the pixel of interest and results obtained by multiplying adjacent pixel signals 808 and 809 of the main scanning direction with -1. Therefore, a quadratic differential signal 812 for the pixel of interest in the main scanning direction is obtained.
An adder 815 adds results obtained by multiplying pixel signals 813 and 814 of lines adjacent to the line including the pixel of interest by -1 and a result obtained by doubling the signal 807 of the pixel of interest. Therefore, a quadratic differential signal 817 for the pixel of interest in the sub-scanning direction is obtained.
These quadratic signals 812 and 817 are added to the signal 807 of the pixel of interest by an adder 818 to obtain an edge-emphasized image signal 819.
An output image is adversely affected by the above-mentioned variable magnification techniques and edge emphasis processing in various ways.
First, even if an original having a uniform density is read, an amplitude of the resultant image signal is not uniform partially due to an internal arrangement of a CCD line sensor, as shown in FIG. 2.
Even- and odd-numbered pixel outputs from light-receiving cells 601 are alternately transferred to separate charge transfer units 602 and 603 and are output as a one-line image signal at a multiplexer 606 through corresponding amplifiers 604 and 605.
Variations of digital image signals from the pixels depend on variations in sensitivity of the light-receiving cells, DC offset variations caused by different charge transfer units, and nonlinear gains for small signals in the amplifiers.
In order to correct the above variations, various correction techniques such as DC drift elimination and shading correction have been proposed. However, these conventional techniques are based on an assumption that a CCD line sensor output is proportional to an amount of light received by the CCD line sensor. Therefore, the above correction cannot be performed with high precision due to nonlinearity of the light-receiving element with respect to a very small amount of input light and nonlinearity of the amplifier.
This correction error is primarily included in black information and is caused by variations in units of pixels as shown in FIG. 3A. The variations in the main scanning direction are emphasized by the edge emphasis circuit (FIG. 1), as shown in FIG. 3B (compare the magnitude of original difference X with the difference Y which results after edge emphasis).
The above variations are further emphasized by image thinning for image reduction, as shown in FIG. 3C. A portion such as the C-1 portion obtained by thinning a portion having a high density consists of successive bright pixels and is emphasized as a sharp white line in a copy output. A portion such as the C-2 portion obtained by thinning a portion having a low density consists of successive dark pixels and is emphasized as a sharp black line in a copy output.
In addition, when an image is enlarged, edge-emphasized image information is increased along the main scanning direction, as shown in FIG. 3D. Variations in density are emphasized by a degree corresponding to an increased output area per pixel.
As is apparent from the above description, CCD variations emphasized by edge emphasis in the conventional apparatus are further emphasized by variable magnification processing along the main scanning direction, resulting in inconvenience.
Mismatching between variable magnification processing and edge emphasis processing in the sub-scanning direction also results in inconvenience.
As shown in FIG. 4A, one pixel of a CCD line sensor has a predetermined length in both the main scanning and the sub-scanning directions. The predetermined length is represented by length a. When a CCD line sensor with the opening length a is shifted by a distance b with respect to the original in the sub-scanning direction and one pixel is scanned and read, an area of a .times.(a+b) is read as one pixel, as shown in FIG. 4B.
Assume that the read distance b in the sub-scanning direction is defined as a scanning length in a one-to-one reading. As shown in FIG. 4C, an image read with a scanning line in an original area S1 is recorded as a pixel P1 in the printer. An image read with the same pixel on the next line in an original area S2 is recorded as a pixel P2 in the printer. The pixels P1 and P2 include a common blurring portion (indicated by hatched lines) corresponding to the opening area of the CCD line sensor.
A ratio of blurring per pixel of the recorded image is given as a/(a+b).
As shown in FIG. 4D, where original areas S3 and S4 are recorded respectively as pixels P.sub.3 and P.sub.4, if an original read displacement per pixel in the sub-scanning direction is given as b/4 and a recording magnification in the sub-scanning direction is given as 400%, the ratio of blurring per pixel of the recorded image is given as a/(a+b/4). In this manner, when an enlargement coefficient is increased, the sub-scanning length is reduced. A decrease in the value of the denominator of the blurring ratio expression causes an increase in blurring.
In a conventional apparatus, when sub-scanning edge emphasis having a fixed magnitude is used to increase the enlargement coefficient, the amount of blurring included in the image is undesirably increased.